Groundwater, a valuable and limited natural resource, can become contaminated by volatile organic compounds (VOC) and semi volatile organic compounds (SVOC) by: (i) leaking underground storage tanks and associated piping (e.g., gasoline stations); (ii) leaking/ruptured pipelines; (iii) chemical spills along roadways, at chemical plants, or manufacturing operations; and (iv) leaching of chemicals disposed of in landfills.
Chemicals spilled as described above, if not immediately cleaned up, can be absorbed into the soil, subsequently transported (depending on the solubility of the contaminant) via rainwater to underground aquifers. Once in the aquifer, the contaminants spread and are carried down gradient. This spreading and movement of the contaminants is known as a “plume”. Drinking water wells, buildings, wetlands, etc. which are down gradient of the spill site can be negatively impacted by the contaminant plume, posing health risks to wildlife and to humans.
The treatment of contaminated soils and groundwater has gained increased attention over the past few years because of uncontrolled hazardous waste disposal sites. It is well documented that the most common means of site remediation has been excavation and landfill disposal. While these procedures remove contaminants, they are extremely costly and in some cases difficult if not impossible to perform.
More recently, research has focused on the conversion of contaminants contained in soil and groundwater based on the development of on-site and in situ treatment technologies. One such treatment has been the incineration of contaminated soils. The disadvantage of this system is in the possible formation of harmful by products including polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF).
In situ biological soil treatment and groundwater treatment is another such system that has been reviewed in recent years. So-called bioremediation systems, however, have limited utility for treating waste components that are biorefractory or toxic to microorganisms.
Such bioremediation systems were the first to investigate the practical and efficient injection of hydrogen peroxide into groundwater and/or soils. These investigations revealed that the overriding issue affecting the use of hydrogen peroxide in situ was the instability of the hydrogen peroxide downgradient from the injection point. The presence of minerals and enzymes such as catalase and peroxidase in the subsurface catalyzed the disproportionation of hydrogen peroxide near the injection point, with rapid evolution and loss of molecular oxygen, leading to the investigation of stabilizers as well as biological nutrients.
During the early biological studies from the 1980's, some investigators recognized the potential for competing reactions, such as the direct oxidation of the substrate by hydrogen peroxide. Certain researchers also hypothesized that an unwanted in situ Fenton's-like reaction under native conditions in the soil was reducing yields of oxygen through the production of hydroxyl radicals, a powerful oxidizing species. Such a mechanism of contaminant reduction was not unexpected, since Fenton's-type systems have been used in ex situ systems to treat soil and groundwater contamination.
Other investigators concomitantly extended the use of Fenton's-type systems to the remediation of in situ soil systems. These studies attempted to correlate variable parameters such as hydrogen peroxide, iron, phosphate, pH, and temperature with the efficiency of remediation.
As with bioremediation systems, in situ Fenton's systems were often limited by instability of the hydrogen peroxide in situ and by the lack of spatial and temporal control in the formation of the oxidizing agent (i.e. hydroxyl radical) from the hydrogen peroxide. In particular, aggressive/violent reactions often occurred at or near the point where the source of the oxidizing agent (the hydrogen peroxide) and the catalyst were injected. As a consequence, a significant amount of reagents including the source of the oxidizing agent (hydrogen peroxide) was wasted because activity was confined to a very limited area around the injection point. In addition, these in situ Fenton's systems often required the aggressive adjustment of groundwater pH to acidic conditions, which is not desirable in a minimally invasive treatment system. Finally, such systems also resulted in the mineralization of the subsurface, resulting in impermeable soil and groundwater phases due to the deleterious effects of the reagents on the subsurface soils.
Other researchers have investigated the use of ozone, either alone or in combination with hydrogen peroxide, in ex situ advanced oxidation processes (AOPs) wherein ozone (O3) and hydrogen peroxide (H2O2) introduced into water react with each other to form the hydroxyl radical (HO*). The hydroxyl radical formation reaction is as follows:H2O2+2O3→2OH*+3O2  (1)Hydrogen peroxide, ozone, and hydroxyl radical then come into contact with and oxidize contaminants, destroying them. Glaze and Kang, J. Amer. Water Works Assoc., 80, 51 (1988), is hereby incorporated by reference in its entirety, describes an advanced oxidation process wherein ozone (O3) and hydrogen peroxide (H2O2) are introduced into contaminated water at atmospheric pressure.
Known AOP decontamination systems suffer from a number of disadvantages. A first disadvantage of known AOP decontamination systems is formation of unwanted disinfection byproducts. For example, bromide ions (Br−), naturally present in the water, can undergo a series of reactions to produce bromate (BrO3−):3Br−+O3(only)→3BrO−  (2)BrO+(O3 or HO*)→BrO3−  (3)Bromate has recently been designated as a suspected carcinogen, and the U.S.E.P.A. has established a maximum level for drinking water of 10 μg/L. It is thus important to prevent or minimize bromate formation during decontamination of potable water.
In reaction (2) above, neither the hydroxyl radical (HO*) nor hydrogen peroxide alone oxidize bromide to form hypobromite (BrO−). Moreover, reaction (3) must compete with the conversion of hypobromite back to bromide that occurs in the presence of hydrogen peroxide:BrO−+H2O2→Br−  (4)Thus when hydrogen peroxide concentration is greater, reaction (4) is favored and the formation of bromate is discouraged. See U. von Gunten and Y. Oliveras, Envir. Sci. and Tech., 32, 63 (1998); U. von Gunten, Y. Oliveras, Wat. Res., 31, 900 (1997); W. R. Haag, and J. Hoigne, Envir. Sci. and Tech., 17, 261(1983); U. von Gunten, J. Hoigne and A. Bruchet, Water Supply, 13, 45 which are all hereby incorporated by reference in their entireties.
A second disadvantage of conventional ozone decontamination systems is the limited solubility of ozone in water at atmospheric pressure. FIG. 1 shows that the solubility of ozone in water increases with higher pressure. However, conventional oxidation decontamination systems introduce ozone at only atmospheric pressure, limiting the amount of ozone that can be dissolved in the water.
A third disadvantage is the limited concentration of ozone normally present in the reactant gas stream that is mixed with the water. FIG. 2 shows that ozone solubility in water increases with increasing ozone in the gas phase. Conventional oxidation systems utilize gas streams containing only about 1-4% ozone by weight in air, effectively limiting the amount of ozone soluble in water.
Finally, these AOP decontamination systems suffer from a similar limitation as all ex situ systems; namely, the necessity to pump contaminants from the in situ media to an external reaction vessel, a requirement which is both expensive and inefficient.